1. Field of the Invention
This invention relates to apparatus for use in the detection of changes in variables, e.g. liquid leaks.
2. Introduction to the Invention
A number of methods have been used (or proposed for use) to detect changes in variables at a point or points or along an elongate path, or on a surface, or in a zone, e.g. the occurrence of a leak (of water or another liquid or gas), insufficient or excessive pressure, too high or too low a temperature, the presence or absence of light or another form of electro-magnetic radiation, or a change in the physical position of a movable member, e.g. a valve in a chemical process plant or a window in a building fitted with a burglar alarm system. Changes of this kind are referred to in this specification by the generic term "event". Reference may be made for example to U.S. Pat. Nos. 1,084,910, 2,581,213, 3,248,646, 3,384,493, 3,470,340, 3,800,216, 3,991,413, 4,278,931 and 4,400,996, U.K. Pat. Nos. 1,481,850, 1,355,176 and 182,339, German Offenlegungschriften Nos. 3,001,150.0 and 3,225,742, and copending, commonly assigned, U.S. patent applications Ser. Nos. 691,291 (now abandoned), 831,758 (now abandoned) in favor of continuation-in-part application Ser. No. 017,375), 832,562, 838,725, and 856,925 (now abandoned). The disclosures of each of the patents, applications and other publications referred to above are incorporated herein by reference.